DESCRIPTION: (Scanned from the applicant?s abstract) An important problem of cell biology concerns how the cell-cell interaction between egg and sperm stimulates Ca2+ release from the egg?s endoplasmic reticulum, leading to reinitiation of the cell cycle. In mammals and ascidians, injection of a cytosolic sperm extract can also stimulate Ca2+ release in the egg. In ascidians, sperm extract injection has been shown to stimulate Ca2+ release via the same signaling pathway used at fertilization. In this pathway, an Src family kinase in the egg directly or indirectly activates phospholipase Cg, which produces the inositol triphosphate that stimulates Ca2+ release from the egg?s endoplasmic reticulum. This result suggests that, at fertilization, the Ca2+ release pathway is initiated during sperm-egg fusion when an activating protein is introduced from the sperm into the egg. The major goals of this proposal are to isolate a cDNA encoding for Src from an ascidian egg cDNA library and to use the ascidian egg Src as a tool to isolate the putative activating protein from the sperm extract. Isolating a sperm protein that acts upstream of Src in the signaling pathway that leads to egg activation will not only provide information about this cellular signaling pathway, but may also contribute to the understanding of how Src family kinases are activated during cell-cell interactions and how these kinases function in cell cycle control.